Redox state of NAD modulates the activation of Na-bicarbonate cotransporter NBCe1-B via IRBIT and L-IRBIT.

Abstract

Nicotinamide adenine dinucleotide (NAD) is well known as a coenzyme involved in many redox reactions in cellular energy metabolism, or as a substrate for many NAD⁺-consuming enzymes, including those that generate the second messenger cyclic ADP-ribose or deacetylate proteins (e.g., histones). The role of NAD in non-catalytic proteins is poorly understood. IRBIT and L-IRBIT (the IRBITs) are two cytosolic proteins that are structurally related to dehydrogenases but lack catalytic activity. Instead, by interacting directly with their targets, the IRBITs modulate the function of numerous proteins with important roles, ranging from Ca²⁺ signaling and intracellular pH (pH_i) regulation to DNA metabolism to autophagy. Among the targets of the IRBITs is the Na⁺-HCO₃^- cotransporter NBCe1-B, which plays a central role in intracellular pH (pH_i) regulation and epithelial electrolyte transport. Here, we demonstrate that NAD modulates NBCe1-B activation by serving as a cofactor of IRBIT or L-IRBIT. Blocking NAD salvage pathway greatly decreases NBCe1-B activation by the IRBITs. Administration of the oxidized form NAD⁺ enhances, whereas the reduced form NADH decreases NBCe1-B activity. Our study represents the first example in which the redox state of NAD, via IRBIT or L-IRBIT, modulates the function of a membrane transport protein. Our findings reveal a new role of NAD and greatly expand our understanding of NAD biology. Because the NAD redox state fluctuates greatly with metabolic status, our work provides insight into how, via the IRBITs, energy metabolism could affect pH_i regulation and many other IRBIT-dependent processes.